Primary hyperparathyroidism (PHP) in the pregnant patient can pose significant risks to the mother and fetus. The actual incidence of PHP in the pregnant population is unknown as the subtle symptoms and findings can often be masked by the physical and physiologic changes that occur with pregnancy. The presentation can range from asymptomatic to severely symptomatic. Symptoms in the mother can include all the typical symptoms of PHP such as nephrolithiasis, dehydration, pancreatitis, bone disease, and hypercalcemic crisis as well as hyperemesis gravidarum. Complications from maternal PHP affecting the developing fetus can include spontaneous abortion, preterm delivery, postpartum neonatal tetany, and permanent hypoparathyroidism. The prompt diagnosis of this condition can prevent many of these complications. The following chapter will discuss the physiology of calcium homeostasis between the mother and fetus, diagnosis of PHP, localization studies, as well as the surgical and medical treatment options.